A Criterion for the Preliminary Design of High-Efficiency Tube-Axial Fans

Abstract

Tube-axial fans are widely used in industrial applications because of their compactness, simplicity, and low cost. However, the achievable fan pressure rise is generally penalised by the absence of a straightener and diffuser, and the consequent waste of tangential and axial dynamic pressures at the fan outlet. The corresponding fan efficiency drop might not comply with stringent regulations like the European Directive for energy-related products. Thus, operation ranges of high efficiency need to be clearly defined in the preliminary design phase, especially when constraints on maximum size and/or rotational speed are imposed. This paper proposes analytical formulas and charts to evaluate the efficiency of the tube-axial fan configuration (with or without tail-cone diffuser) when constraints on fan size and/or speed are additional design requirements. The analytical formulas and charts have been validated against experimental data. On this basis, a preliminary design criterion is suggested for high-efficiency tube-axial fans featuring arbitrary vortex design blades of constant swirl type. The criterion is used to design a 315 mm low-to-medium pressure tube-axial fan that is able to operate at a constant aeraulic efficiency peak of approximately 0.6 for blade positioning angles in the range 20° to 30°.

Design for high efficiency of low-pressure axial fans: Use of blade sweep and vortex distribution

“…Optimal Vortex Distribution Design Method axial velocity, and hence reduced hydraulic losses. A similar conclusion has been reported by Masi et al [124]. By reducing the axial velocity, it is expected that η ts.max increases due to decreased exit losses (dynamic pressure).…”

“…Masi and Lazzaretto [123] in 2019 described design approach, integrating design ideas reported by Wallis et al [21,121] and Vad [122] for design of axial fans with high total-to-total efficiency η tt , where a mean-line model, described in Ref. [124], was applied to estimate the η tt of axial fans having constant swirl blade loading. The results showed that the design method permits to obtain increased performance with respect to η tt , but the low Reynolds number due to the fan's small size limited the improvement.…”

“…A preliminary design guideline based on the achievable total-to-total efficiency η tt was reported by Masi et al [124] where the fan performance, including considerations of inlet and outlet guide vanes, were analysed theoretically with a linear vortex distribution (corresponding to a constant local swirl coefficient v θ2 /v x ). An efficiency chart as function of flow coefficient ϕ, swirl coefficient and HTR was presented.…”

“…The previous studies [13,123,124,145] have shown that in order to increase the total-to-static efficiency of rotor-only fans, the HTR κ may be reduced, since this gives smaller wasted axial kinetic energy. However, for fans with small κ backflow is more likely to occur according to the well-known Strscheletzky criterion [8,149,150].…”

Computational characterization of an axial rotor fan

“…In Ref. [3],the authors proposed several analytical formulas and charts to calculate the efficiency of different axial fan configurations on a tube. Matai et al [2] proposed quite a few design parameters to evaluate the performance of an axial fan using Computational Fluid Dynamics CFD techniques.…”